The Islamic (Hijri) Calendar

The Islamic calendar is based on the lunar year of about 354 days. The
number of days each month is adjusted according to the lunar cycle,
beginning about two days after the new moon. The months drift backward
over the seasons, returning to their starting points after about 33 lunar
years. The Islamic year begins on the first day of Muharram, and is
counted from the year of the Hegira (anno Hegirae)—the year in
which Muhammad emigrated from Mecca to Medina (A.D. 622). The year 2015 translates to A.H. 1436–1437. The year 2016 translates to A.H. 1437–1438.